mlxsw: spectrum: Cosmetic naming change

Currently the struct representing router interface "mlxsw_sp_rif"
is reffered as "r" in various places in the driver. Furthermore it
contains a member which specify the index which is called "rif".
This patch change "r" to "rif" and "rif" to "rif_index".

Signed-off-by: Arkadi Sharshevsky <arkadis@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>